tor-spec: Add authenticated SENDMEs

This merges proposal 289 into tor-spec.txt.

Most of the circuit-level flow control section has been rewritten to be
clearer and better detail version 0 and 1.

Closes #30365

Signed-off-by: David Goulet <dgoulet@torproject.org>

1 files changed, 88 insertions, 20 deletions

diff --git a/tor-spec.txt b/tor-spec.txt
index b2b26eb..1a211bc 100644
--- a/tor-spec.txt
+++ b/tor-spec.txt
@@ -1861,32 +1861,91 @@ see tor-design.pdf.
 
    To control a circuit's bandwidth usage, each OR keeps track of two
    'windows', consisting of how many RELAY_DATA cells it is allowed to
-   originate (package for transmission), and how many RELAY_DATA cells
-   it is willing to consume (receive for local streams).  These limits
-   do not apply to cells that the OR receives from one host and relays
-   to another.
+   originate or willing to consume.
+
+   These two windows are respectively named: the package window (packaged for
+   transmission) and the deliver window (delivered for local streams).
+
+   Because of our leaky-pipe topology, every relay on the circuit has a pair
+   of windows, and the OP has a pair of windows for every relay on the
+   circuit. These windows do not apply to relayed cells, however, and a relay
+   that is never used for streams will never decrement its window or cause the
+   client to decrement a window.
 
    Each 'window' value is initially set based on the consensus parameter
-   'circwindow' in the directory (see dir-spec.txt), or to 1000 data cells
-   if no 'circwindow' value is given,
-   in each direction (cells that are not data cells do not affect
-   the window).  When an OR is willing to deliver more cells, it sends a
-   RELAY_SENDME cell towards the OP, with Stream ID zero and an empty
-   body. When an OR receives a RELAY_SENDME cell with stream ID zero, it
-   increments its packaging window. (The body SHOULD be ignored.)
+   'circwindow' in the directory (see dir-spec.txt), or to 1000 data cells if
+   no 'circwindow' value is given. In each direction, cells that are not
+   RELAY_DATA cells do not affect the window.
+
+   An OR or OP (depending on the stream direction) sends a RELAY_SENDME cell
+   to indicate that it is willing to receive more cells when its deliver
+   window goes down below a full increment (100). For example, if the window
+   started at 1000, it should send a RELAY_SENDME when it reaches 900.
+
+   When an OR or OP receives a RELAY_SENDME, it increments its package window
+   by a value of 100 (circuit window increment) and proceeds to sending the
+   remaining RELAY_DATA cells.
+
+   If a package window reaches 0, the OR or OP stops reading from TCP
+   connections for all streams on the corresponding circuit, and sends no more
+   RELAY_DATA cells until receiving a RELAY_SENDME cell.
+
+   If a deliver window goes below 0, the circuit should be torn down.
+
+   Starting with tor-0.4.1.1-alpha, authenticated SENDMEs are supported
+   (version 1, see below). This means that both the OR and OP need to remember
+   the rolling digest of the cell that precedes (triggers) a RELAY_SENDME.
+   This can be known if the package window gets to a multiple of the circuit
+   window increment (100).
+
+   When the RELAY_SENDME version 1 arrives, it will contain a digest that MUST
+   match the one remembered. This represents a proof that the end point of the
+   circuit saw the sent cells. On failure to match, the circuit should be torn
+   down.
+
+   To ensure unpredictability, random bytes should be added to at least one
+   RELAY_DATA cell within one increment window. In other word, every 100 cells
+   (increment), random bytes should be introduced in at least one cell.
+
+7.3.1 SENDME Cell Format
+
+   A circuit-level RELAY_SENDME cell always has its StreamID=0.
 
-   Each of these cells increments the corresponding window by 100.
+   An OR or OP must obey these two consensus parameters in order to know which
+   version to emit and accept.
 
-   The OP behaves identically, except that it must track a packaging
-   window and a delivery window for every OR in the circuit.
+      'sendme_emit_min_version': Minimum version to emit.
+      'sendme_accept_min_version': Minimum version to accept.
 
-   An OR or OP sends cells to increment its delivery window when the
-   corresponding window value falls under some threshold (900).
+   If a RELAY_SENDME version is received that is below the minimum accepted
+   version, the circuit should be closed.
 
-   If a packaging window reaches 0, the OR or OP stops reading from
-   TCP connections for all streams on the corresponding circuit, and
-   sends no more RELAY_DATA cells until receiving a RELAY_SENDME cell.
-[this stuff is badly worded; copy in the tor-design section -RD]
+   The RELAY_SENDME payload contains the following:
+
+      VERSION     [1 byte]
+      DATA_LEN    [2 bytes]
+      DATA        [DATA_LEN bytes]
+
+   The VERSION tells us what is expected in the DATA section of length
+   DATA_LEN and how to handle it. The recognized values are:
+
+      0x00: The rest of the payload should be ignored.
+
+      0x01: Authenticated SENDME. The DATA section MUST contain:
+
+         DIGEST   [20 bytes]
+
+         If the DATA_LEN value is less than 20 bytes, the cell should be
+         dropped and the circuit closed. If the value is more than 20 bytes,
+         then the first 20 bytes should be read to get the DIGEST value.
+
+         The DIGEST is the rolling digest value from the RELAY_DATA cell that
+         immediately preceded (triggered) this RELAY_SENDME. This value is
+         matched on the other side from the previous cell sent that the OR/OP
+         must remember.
+
+   If the VERSION is unrecognized or below the minimum accepted version (taken
+   from the consensus), the circuit should be torn down.
 
 7.4. Stream-level flow control
 
@@ -2111,3 +2170,12 @@ see tor-design.pdf.
    Describes the padding capabilities of the relay.
 
    "1" -- Relay supports circuit-level padding.
+
+9.12. "FlowCtrl"
+
+   Describes the flow control protocol at the circuit and stream level. If
+   there is no FlowCtrl advertised, tor supports the unauthenticated flow
+   control features (version 0).
+
+   "1" -- supports authenticated circuit level SENDMEs as of proposal 289 in
+          Tor 0.4.1.1-alpha.